The present invention relates to satellite navigation and positioning systems. More particularly, the present invention relates to a pseudo-satellite (pseudolite) navigation system which enhances operation of a global navigational satellite system (GNSS) when satellite ephemeris data is unavailable.
Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency f1 of 1575.42 MHz, and an L2 signal having a frequency f2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code is reserved for military uses.
Conventional GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver.
By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). A conventional GPS receiver typically utilizes the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. The GPS signals also include a 50 bit per second data stream or data message which is superimposed on the C/A and Y-codes. Once the receiver has matched its code to the code in the GPS signal from a particular satellite, the receiver can decipher the data message. The data message can include navigational data related to the position of the satellite, including geometric dilution of precision (GDOP) parameters. Additionally, the data message can include accurate time data, ephemeris data, and data related to the health status of the satellite. The GPS satellites utilize code division multiple access techniques so satellite signals do not interfere with each other. GLONASS navigational systems operate similarly to GPS navigational systems and utilize frequency division multiple access (FDMA) techniques so satellite signals do not interfere with each other.
GNSS navigational systems have tremendous benefits over other positioning and navigational systems because these systems do not rely upon visual, magnetic or other points of reference. However, conventional GNSS navigational systems are susceptible to jamming by higher power signals. When jammed by higher power signals, the C/A-code encoded signal is difficult to acquire. Typically C/A-code acquisition is necessary to acquire the Y-code signal. Therefore, in the presence of jamming signals, navigation using a GNSS receiver can prove problematic.
Disclosed are methods of transferring satellite ephemeris data for a first GNSS satellite from a pseudolite to a GNSS receiver. Also disclosed are pseudolites and GNSS receivers implementing the same. A first satellite global positioning signal transmitted by the first GNSS satellite is received at the pseudolite. The pseudolite transmits a pseudolite global positioning signal containing the satellite ephemeris data for the first GNSS satellite. The GNSS receiver receives the pseudolite global positioning signal and retrieves from the pseudolite signal the satellite ephemeris data.